This invention relates to a keyboard control system for controlling the input operation to a keyboard.
Generally, a keyboard is connected to a keyboard controller which responds to the depression of a key on the keyboard by generating a key matrix code corresponding to the depressed key, and by supplying the code to a CPU or the like. When an ordinary keyboard controller detects a fresh key depression, it validates a key matrix code related to the operated key and stores it in a keyboard buffer memory. The keyboard controller, if it has a repeat function, repeatedly stores the key matrix code into the keyboard buffer memory at fixed intervals, for as long as the key depression continues. Thus, the keyboard controller validates the key matrix code whenever the key is depressed, or when its depression continues for an extended period.
However, the keyboard controller cannot invalidate the key matrix code after it has been validated, even if the key corresponding to the valid code is released from its depressed state. After a key matrix code has been validated in the prior keyboard controller, it cannot be invalidated until another key is depressed. With respect to ordinary function keys, the continuing validity of the key matrix code does not create a problem. With respect to a special function key contained in the keyboard of computer terminal equipment for the function of displaying a guidance, this undeterred validity creates a problem such that the guidance display cannot be extinguished.
To cope with this problem, a lock key capable of invalididating the key matrix code and accommodated with an alternate mechanism has recently been developed. This key operates such that the first depression of the key mechanically locks the key, while the second depression releases the key from its locked state. When it is locked, a key matrix code associated with the key is validated. When it is unlocked, the code is invalidated. Unfortunately, however, this type of key has some disadvantages. For example, the mechanism to lock the key is complicated in structure and therefore expensive to manufacture. Further, double key actions are required for locking and unlocking operations, therby making the keyboard operation time consumming and laborious.